The present invention relates, in general, to semiconductor processing methods, and more particularly, to a novel way of forming thin epitaxial layers from a liquid phase solution.
The semiconductor industry has long used crystallization of semiconductor material from a heated liquid phase solution to form epitaxial layers on semiconductor wafers. One particular use was to grow P-type and N-type epitaxial layers on gallium arsenide (GaAs) wafers. The epitaxial layers typically were grown by using a graphite boat which exposed a number of wafers to a melt that contained a solvent of melted semiconductor material such as gallium (Ga), into which was dissolved solutes, such as melted gallium arsenide, and dopants, such as melted silicon. The wafers were pushed into the melt thereby covering them with a thick layer, generally greater than 1000 microns, of the melt. While the wafers were covered, the temperature of the boat and melt was decreased which caused crystallization of material from the melt onto the wafers thereby growing epitaxial layers of the melt material onto the wafers. Cooling of the boat continued until it was cool enough for unloading the wafers. These techniques produced thick epitaxial layers, generally 200 microns or greater, which was thicker than the desired layers. Consequently, the wafers were polished by mechanical and/or chemical means to obtain the desired thin epitaxial layers. Such polishing was time consuming and increased the costs of producing liquid phase epitaxial layers. Polishing operations often broke the brittle gallium arsenide wafers which resulted in reduced yield of the manufacturing process and further increased manufacturing costs. Polishing of the wafers also roughened the epitaxial layer's surface thereby reducing the yield of subsequent processing operations which further increased manufacturing costs. Additionally, wafer polishing can produce stress on the semiconductor material's surface thereby reducing semiconductor device performance and reliability.
Accordingly, it is desirable to have a liquid phase epitaxial process that produces thin epitaxial layers, that reduces wafer breakage, that produces thin epitaxial layers having surfaces suitable for subsequent processing operations, and that reduces the manufacturing costs of producing thin epitaxial layers.